These springs allow your Classic Mustang to take advantage of the latest suspension technology and perform more like a modern sports car. They really do give you the best of both worlds. A fairly stock smooth ride during normal driving, but as the springs compress under load (primarily during braking and cornering) they progressively become firmer very quickly to offer the handling characteristics similar to our 600 pound performance springs, but without sacrificing ride comfort. Made in the USA by a premier OEM supplier (for strictly race car applications we still recommend our Performance Springs). Our Progressive Rate Coil Springs feature: Hot wind from SAE5160H, oil quench, temper, water quench, shot peen, preset, 100% load test, ground flat and black powder coating.